Re: [PATCH v7 00/13] fold per-CPU vmstats remotely

[Marcelo Tosatti]

On Mon, Mar 20, 2023 at 07:25:55PM +0100, Michal Hocko wrote:
> On Mon 20-03-23 15:03:32, Marcelo Tosatti wrote:
> > This patch series addresses the following two problems:
> > 
> > 1. A customer provided evidence indicating that a process
> >    was stalled in direct reclaim:
> > 
> This is addressed by the trivial patch 1.
> 
> [...]
> >  2. With a task that busy loops on a given CPU,
> >     the kworker interruption to execute vmstat_update
> >     is undesired and may exceed latency thresholds
> >     for certain applications.
> 
> Yes it can but why does that matter?

It matters for the application that is executing and expects
not to be interrupted.

> > By having vmstat_shepherd flush the per-CPU counters to the
> > global counters from remote CPUs.
> > 
> > This is done using cmpxchg to manipulate the counters,
> > both CPU locally (via the account functions),
> > and remotely (via cpu_vm_stats_fold).
> > 
> > Thanks to Aaron Tomlin for diagnosing issue 1 and writing
> > the initial patch series.
> > 
> > 
> > Performance details for the kworker interruption:
> > 
> > oslat   1094.456862: sys_mlock(start: 7f7ed0000b60, len: 1000)
> > oslat   1094.456971: workqueue_queue_work: ... function=vmstat_update ...
> > oslat   1094.456974: sched_switch: prev_comm=oslat ... ==> next_comm=kworker/5:1 ...
> > kworker 1094.456978: sched_switch: prev_comm=kworker/5:1 ==> next_comm=oslat ...
> >  
> > The example above shows an additional 7us for the
> > 
> >         oslat -> kworker -> oslat
> > 
> > switches. In the case of a virtualized CPU, and the vmstat_update
> > interruption in the host (of a qemu-kvm vcpu), the latency penalty
> > observed in the guest is higher than 50us, violating the acceptable
> > latency threshold for certain applications.
> 
> I do not think we have ever promissed any specific latency guarantees
> for vmstat. These are statistics have been mostly used for debugging
> purposes AFAIK. I am not aware of any specific user space use case that
> would be latency sensitive. Your changelog doesn't go into details there
> either.

There is a class of workloads for which response time can be
of interest. MAC scheduler is an example:

https://par.nsf.gov/servlets/purl/10090368

Thanks!